The Spallation Neutron Source, one of ORNL’s signature research facilities, resumed operations a week ago following an abbreviated winter maintenance outage. According to Kevin Jones, who heads the lab’s Research Accelerator Division, the SNS is operating well at a reduced power level of 850 kW, which he acknowledged was a “target conservation mode.”
A couple of the target vessels failed prematurely last fall, which created troubling interruptions in the schedule of research experiments. The target vessel holds and circulates 20 tons of mercury, which produces neutrons when struck many times a second by a powerful proton beam. The SNS is running at below-max power to help protect the target vessel currently installed until more backups arrive at the Oak Ridge complex.
The SNS operations team is busy investigating what caused Target 10 and Target 11 to fail unexpectedly. Jones said investigators have identified the location of the failure on both target vessels, but the actual cause is a still a matter of review and conjecture.
Jones said the winter maintenance outage at the Spallation Neutron Source was shortened by two weeks in order to help recover some of the research time that was lost last fall. Operations resumed Jan. 23. “Availability for users from that time to the present is 93.5 percent, ahead of our goal of 90 percent,” he said via email.
A second spare target vessel is expected to arrive in late March, and the SNS team is planning a one-week maintenance period beginning April 13. Neutron production for experiments is due to resume on April 21.
“Assuming that the current target continues to operate well and that we have two certified spares available, we plan to raise the machine power to approximately 1.2 MW or above when we resume operation on April 21st,” Jones said.
The ORNL official provided some detailed descriptions of the evaluations to date on Target 10 and Target 11.
“For Target 11 the failure was in the same general location as the two targets that failed in 2012, but the weld was of much better quality. So this is a little puzzling, and we are developing procedures to try to cut out the failure location and enough surrounding material to permit a careful analysis of the failure itself. We did not do this on the targets that failed in 2012 because the defective nature of the weld was much more obvious in those cases. We have also taken the standard samples from the nose of the target for the usual evaluation of the progression of the cavitation damage.
“For Target 10, the failure was also at a weld joint, but in a different location on the upper surface of the mercury vessel. Similar to Target 11, the weld quality was deemed good. We are likewise developing procedures to cut out the failed region with enough surrounding material to permit analysis. We have also taken the standard samples from the nose of the target for evaluation of the cavitation damage.The interesting point here is that this target, the first ‘jet flow’ design, had an irradiation history very similar to one of the first standard flow targets that failed prematurely in 2012, Target 6. This will allow us to make direct comparisons between the internal cavitation damage patterns for a standard flow and a jet flow target. This analysis is ongoing, but preliminary results indicate that the jet-flow concept (created to help extend the lifetime of the target vessel) does indeed reduce the internal cavitation damage as intended.”
Jones said analysis and modeling of the target design and the stresses experienced during operations are ongoing as well.
“We will have an external group of experts visit us in the last week of February to review our analyses of the failures and the analytical modeling we are doing, and we expect that this group will provide us with both a critical review of the work we have done and valuable advice on other possible avenues of assessment to improve our understanding,” he said.
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